Fourth nerve palsy refers to a congenital defect or an acquired injury to the fourth cranial nerve, which is responsible for eye movement. When the nerve is damaged or malformed, the superior oblique muscle in the skull behind the eye cannot keep it aligned straight ahead. The affected eye tends to drift vertically, horizontally, or both from center, causing double vision that can only be relieved by tilting the head to one side. Vision problems and self-consciousness about the problem can significantly impair a person's ability to engage in normal daily tasks. Surgery is typically required to correct eye positioning and minimize vision stress.
Most cases of fourth nerve palsy are the result of developmental abnormalities in pregnancy. The causes of congenital palsy are not well understood, and there are no clear connections between palsy and a mother's medication use, diet, or other environmental factors. When fourth nerve palsy develops later in life, it is usually due to a major head injury after a fall from height or a high-impact car crash. The condition can be either unilateral, or confined to one side, or bilateral, affecting both eyes. Most cases of both congenital and acquired fourth nerve palsy are unilateral.
The deviation of the eye from center alignment is pronounced in some people and very subtle in others. When the eye does not appear to be misaligned, the primary sign of fourth nerve palsy is a tendency to tilt the head to one side. An infant or child can be observed tilting his or her head to compensate for distorted double vision, for example. Head tilt is usually consistent with unilateral palsy, though an infant with bilateral problems may adjust his or her head frequently to try to even out vision.
A doctor can diagnose fourth nerve palsy in an infant by evaluating the physical appearance of the eye and determining the degree of head tilt. Imaging scans and electroencephalograms may be taken as well to look for abnormalities in muscles, nerves, and other structures in the skull. Accident victims are evaluated similarly either before or after treatment for other injuries. After confirming a diagnosis, a team of physicians and surgeons can discuss treatment options.
In most cases, a badly damaged or dysfunctional nerve cannot be corrected. Surgical repair is focused instead on adjusting the tightness of the superior oblique muscle. A surgeon can sever the muscle and reattach it further down on the eye to help pull the organ into better alignment. Following successful surgery, head tilt and central vision tend to improve significantly.